1. Field of the Invention
A three-gap motor has a “U” shaped rotor, with permanent magnets on the radial inner surfaces and axial surfaces, that rotate over coils positioned radially on a stator.
2. Description of Related Art
At present most commercial motors have radial gaps separating the rotor and stator. Those with permanent magnets have cylindrical rotors attached to a rotating shaft covered by stationary lamination stacks, with the rotor and stator separated by a radial gap. Arc shaped permanent magnets are mounted around the outer periphery of the rotor and face radially outward. Several pairs of magnets of alternating polarity are disposed around the rotor producing rotating magnetic fields within the radial air gap. Stator windings are placed in slots within the stationary lamination stack and loop from one slot to another. Conductor segments within the slots link gap fields to produce torque. Coil segments that are outside the field, called end windings produce no torque. Examples are G. Kasabian (U.S. Pat. No. 4,625,135, issued Nov. 25, 1986) and Leupold et al (U.S. Pat. No. 5,280,209, issued Jan. 18, 1994).
Few commercial motors are of the axial gap type, that employ flat disc shaped rotors and toroidal cores separated by an axial gap. Permanent magnets, that face the stator axially, are mounted on the flat surface of the rotor. Several pairs of flat magnets of alternating polarity generate a rotating magnetic field within the axial air gap. The stator windings are placed within the slots of the toroidal core, and loop from one slot to another. Conductor segments within the slots link the gap field to produce torque. Conductors outside the segments, called end windings, do not produce torque, but do contribute to loss. Examples of such patents are K. Sakai (U.S. Pat. No. 5,619,087, issued Apr. 8, 1997) and Hawsey et al (U.S. Pat. No. 4,996,457, issued Feb. 26, 1991).
Recently, a few patents have combined both approaches and employed both axial gaps and radial gaps to produce torque. Examples are N. Akiyama (U.S. Pat. No. 5,245,270, issued Sep. 14, 1993), Ewing et al (U.S. Pat. No. 5,625,241, issued Apr. 29, 1997), Morohashi et al (U.S. Pat. No. 5,838,079, issued Nov. 17, 1998), Naito et al (U.S. Pat. No. 5,864,197, issued Jan. 26, 1999), Hsu et al (U.S. Pat. No. 5,952,756, issued Sep. 14, 1999) F. Schmider (U.S. Pat. No. 6,232,690, issued May 15, 2001) and Lucidarme et al (U.S. Pat. No. 6,462,449 issued Oct. 8, 2002).
Hsu et al (U.S. Pat. No. 5,952,756, issued Sep. 14, 1999) describe an outer stator type motor. Three sets of permanent magnets are attached at the center of a shaft and three sets of corresponding stators are fastened to a surrounding frame. Two sets of flat permanent magnets create fields in the two axial gaps and one set of permanent magnets around the periphery create fields in one radial gap. The coil windings loop around the slots. Such 3-gap motors however suffer from several disadvantages. These 3-gapped motors in the prior art do not offer high torque density, are very expensive and have poor efficiency and can run hot.
Soghomoniam et al (U.S. Pat. No. 6,891,306, issued May 10, 2005) show another invention that uses 3 gaps, but with an outer rotor. The rotor uses two sets of flat magnets to energize two axial gaps and two sets of peripheral magnets of relatively smaller size to energize the radial gaps. The flat magnet facing one axial gap is polarized North, while that facing another axial gap is polarized South. A set of solenoidal electromagnets with annular windings generate alternating fields that interact with rotating fields to produce the motor torque.